Development of the quasi-direct temperature control type of modulated
induction thermal plasma and its application to plasma temperature recovery
after molecular gas injection
Yasunori Tanaka, Y. Uesaka, Y. Tsubokawa, Y. Uesugi
Faculty of Electrical Engineering & Computer Science, Kanazawa University,
Kakuma, Kanazawa 920-1192, Japan
Abstract: A new type of modulated induction thermal plasma system has
been developed to control the temperature in thermal plasmas. This system
can quasi-directly control the excitation temperature in a thermal plasma to
trace the modulation control signal given externally, for example, of a rectangular waveform and a triangular waveform. The controlled temperature was
successfully changed from 6000 K to 8000 K following the control signal. Using the newly developed system, the temperature recovery test after nitrogen
molecular gas injection to the thermal plasma was conducted to investigate the
robustness of this system against a disturbance.
Coil Current
Temperature
Temperature
1. Introduction
Previously, the authors have originally developed a
pulse modulated induction thermal plasma (PMITP)
system [1] and then an arbitrary-waveform modulated induction thermal plasma (AMITP) system [2]
to control the temperature and the radical densities
in a thermal plasma in time domain. The PMITP
system modulates the coil current amplitude sustaining an induction thermal plasma into a rectangular
waveform, while the AMITP system does it into another arbitrary periodical waveform given externally.
Such a modulated coil current for the PMITP and
AMITP systems provides the periodical modulation
in the thermal plasma temperature. These periodical modulations in thermal plasma temperature actually enable a control of the time-averaged temperature and time evolution in densities of chemical species.
However, these systems do not directly control the
temperature waveform itself.
We have further developed a new system of modulated induction thermal plasmas to enhance the controllability of the temperature and radical densities
in the modulated thermal plasmas [3, 4]. This new
system is named a feedback control type of modulated induction thermal plasma (FBC-MITP). It can
directly control the thermal plasma temperature waveform to trace the given modulation control signal, for
example, of a triangular waveform.
Coil Current
Keywords: Modulated induction thermal plasma, feedback control, temperature control, excitation temperature
0
10
20
30
Time [ms]
40
50
(a) PMITP or AMITP
60
0
10
20
30
Time [ms]
40
50
60
(b) FBC-MITP
Fig. 1. Concept of quasi-direct control of thermal plasma
temperature.
This paper describes a frequency response and a
traceability of the thermal plasma temperature in the
developed FBC-MITP. In addition, the temperature
recovery test after nitrogen gas injection to the thermal plasma was performed to investigate the robustness of the developed FBC-MITP system.
2. Concept of quasi-direct temperature control
of modulated thermal plasmas
Fig. 1 illustrates the concept of quasi-direct control
of thermal plasma temperature. In a conventional
PMITP or AMITP, the coil current is controlled to be
modulated into a given waveform, and then the modulated current changes the thermal plasma temperature, as presented in Fig. 1(a). In this case, the ther-
Rectifier
circuit
DC-DC
converter
DC-RF
inverter
Impedance
Matching
Observed
at10mm below coil-end
3phase
power
source
Opticalfiber
IGBT
AMITP system
PWM
control
Calc.Tex ofAr
DSP
Functiongenerator
Monochromator
Lens
࡮SA
HR320
0709.
0
PMT
Arat703and714nm
Continuum at709nm
Fig. 2. Schematic of feedback control type of modulated induction thermal plasma (FBC-MITP) system.
mal plasma temperature changes more slowly than
the coil current modulation. On the other hand, the
thermal plasma temperature can be modulated into
a given waveform, for example a rectangular waveform, using a feedback control from the temperature
waveform to the coil current waveform, as indicated
in Fig. 1(b). This corresponds to the quasi-direct control of the thermal plasma temperature. Such a direct control of the thermal plasma temperature can
engender the following expectations: Robustness of
modulated thermal plasmas, high quality control of
the thermal plasma temperature, stepped temperature
changes for a fundamental measurement of reaction
rate in high temperature region, automatic control of
input power to thermal plasmas under heavy gas or
powder loading etc.
3. Feedback control system for modulated induction thermal plasmas
Fig. 2 portrays the schematic diagram of a feedback
control type of modulated induction thermal plasma
(FBC-MITP) system. This system combines a spectroscopic observation system, a temperature calculator and a feedback controller within a digital signal
processor (DSP) together with the AMITP system.
The details of the AMITP system can be found in
Ref. [2]. The spectroscopic observation system measures the time variations in the radiation intensities of
Ar atomic lines at wavelengths 703 and 714 nm and
that of the continuum at 709 nm using a polychromator and three photomultiplier tubes. In this paper, the
observation position was set to a position of 10 mm
below the coil end. From these three radiation intenAr is
sities measured, the Ar excitation temperature T ex
evaluated within the DSP in real time. The evaluated
temperature and the reference temperature are feed-
backed to control the coil current using an insulated
gate bipolar transisitor (IGBT).
4. Temperature trace control using FBC-MITP
The temperature tracing test of the FBC-MITP was
conducted to study the frequency response of the FBCMITP. For this aim of the fundamental study, the reference temperature waveform was selected to be a
sinusoidal waveform with an amplitude of 6000 K to
8000 K. The gas flow rate of Ar was set to 80 slpm.
Fig. 3 includes the reference temperature signal
Ar
given (Ref.Temp.), the Ar excitation temperature T ex
in the AMITP without feedback control (W/O FBC).
Ar was measured when only the coil current
This T ex
was controlled to trace the reference sinusoidal sigAr in the FBC-MITP
nal. In addition, the controlled T ex
(W/ FBC) is also presented in Fig. 3. These results
were all measured at a pressure p of 150 torr. The
frequency f of the reference signal was set to 1 Hz,
Ar was con10 Hz and 36 Hz. At f =1 Hz, the T ex
trolled approximately to trace the reference temperature signal with or without feedback control. HowAr waveform without feedback control was
ever, the T ex
shifted in phase from the reference signal, and its
amplitude was decreased. On the other hand, with
Ar waveform was successfully
feedback control, the T ex
controlled to trace the reference signal. At f =36 Hz,
Ar waveform without feedback control hardly
the T ex
follows the reference temperature signal, while the
Ar waveform with feedback control keeps its amT ex
plitude from 6000 to 8000 K.
From these results, the frequency response of the
Ar
T ex waveform was investigated in the temperature
gain G and the phase difference PD between the refAr . The G was deerence signal and the measured T ex
7500
7000
6500
6000
W/ FBC
Ref. Temp.
Temperature [K]
0.0
0.5
8000
1.0
1.5
2.0
2.5
3.0
f = 10 Hz
W/O FBC
7500
7000
6500
6000
W/ FBC
Ref. Temp.
0.00
Temperature [K]
Temperature gain G [dB]
W/O FBC
0.05
0.10
0.15
0.20
8000
0.30
f = 36 Hz
W/O FBC
7500
0.25
7000
6500
6000
W/ FBC
Ref. Temp.
0.00
0.02
0.04
Time [s]
0.06
W/ FBC (60 torr)
W/ FBC (100 torr)
W/ FBC (150 torr)
2
0
-2
-4
-6
-8
-10
-12
-14
-16
1
W/O FBC (60 torr)
W/O FBC (100 torr)
W/O FBC (150 torr)
10
Frequency [Hz]
100
300
Fig. 4. Frequency response in temperature gain of Ar
FBC-MITP.
Phase difference [deg.]
Temperature [K]
f = 1 Hz
8000
0.08
Fig. 3. Controlled Ar expiation temperature in thermal
plasmas. The frequency of the reference temperature was
set to 1, 10 and 36 Hz. The pressure p was 150 torr.
W/ FBC (60 torr)
W/ FBC (100 torr)
W/ FBC (150 torr)
60
0
-60
-120
-180
-240
-300
1
W/O FBC (60 torr)
W/O FBC (100 torr)
W/O FBC (150 torr)
10
Frequency [Hz]
100
300
Fig. 5. Frequency response in phase difference of temperature control of Ar FBC-MITP.
fined as
Controlled temp.
G = 10 log10
Ar excitation temperature [K]
Ar p−p
|T ex
|
p−p
|T ref |
(1)
Arp−p
where T ex
is the peak-to-peak value of the meap−p
Ar
sured T ex waveform, T ref is the peak-to-peak value
of the reference temperature signal, which corresponds
to 2000 K in case of this experiment. The pressure is
also taken as a parameter because increasing pressure generally elevates a difficulty in the temperature
change of thermal plasmas.
Fig. 4 indicates the frequency response of G with
and without feedback control at different pressures.
Without feedback control, the G markedly decreases
with increasing p and f . For a pressure of 150 torr,
for example, G drops around at f =40 Hz with increasing f , and G reaches to −3 dB at 20 Hz. On
the other hand, with feedback control, G has a higher
value than −3 dB even with increasing p and f up to
200 Hz. Similar dependence on p and f can be found
Ar waveof the phase difference PD between the T ex
form measured and the reference temperature signal.
The PD is indicated as functions of f and p in Fig. 5.
Without feedback control, the absolute value of PD
markedly increases with increasing p and f . This increase in |PD| by increasing p and f was alleviated
by using feedback control.
Using the developed FBC-MITP system, the temperature tracing control was made for different waveAr for difform shapes. Fig. 6 shows the controlled T ex
ferent control signals of (a) triangular, (b) inverted
8000
7000
6000 Ref.temp.
(a) Triangular waveform
Controlled temp.
8000
7000
6000
Ref.temp.
(b) Inverted saw-tooth waveform
Controlled temp.
8000
7000
6000
Ref.temp.
0
50
100 150 200 250 300
Time [ms]
(c) Triangular-rectangular waveform
Fig. 6. Controlled temperature waveforms of Ar modulated thermal plasmas.
saw-tooth, and (c) triangular-rectangular waveforms.
The modulation frequency of all the waveforms are
Ar suc10 Hz. It can be seen that the controlled T ex
cessfully traced to the reference temperature signal
for any shape of the reference signals.
5.
Temperature recovery property after N2 gas
injection
To study the robustness of the developed FBC-MITP,
the temperature recovery property in Ar FBC-MITP
was measured after N2 gas injection. Fig. 7 portrays the experimental setup for gas injection. The
Ar at 10 mm below the coil end was controlled to
T ex
6. Conclusion
We newly developed a FBC-MITP system for a detailed control of temperature of modulated thermal
plasmas. The temperature of the FBC-MITP was
successfully controlled to trace different shapes of
control signals. The developed FBC-MITP can also
enhance robustness against a disturbance on the induction thermal plasmas. The developed FBC-MITP
is expected to be applied for advanced thermal plasma
processings such as surface modification, nanoparticle synthesis etc.
Repetition injection of
N2 gas as a disturbance
Pressure
transducer
M
N2
High-speed
valve
Sheath Gas
90mm
Quartz tube
Probe
Ar Ar
Opening signal
RF coil
8turn
14mm
Coil Length
140mm
10mm
Water
Circulation
Spectroscopic
Spectroscopic
observation
ervation position
obs
observation
position
Temperature
Temperature control
control
to
to be fixed
fixed at
at 7000
7000 K.
K.
N2
Fig. 7. Experimental setup for temperature recovery response of FBC-MITP after gas injection.
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Fig. 8. Temperature recovery response of FBC-MITP after N2 gas injection.
Temperature
recovery time [s]
keep 7000 K. The gas flow rate of Ar sheath gas
was fixed at 100 slpm, and the pressure was set to
60 torr. Under this condition, N2 gas was repeatedly
injected through the water-cooled pipe from the top
of the plasma torch. The repeated injection of N2 gas
was done using an electromagnetic valve installed to
the pipe. The opening and closing time were 0.8 s
and 0.2 s, respectively. The pressure in the pipe was
measured to confirm the N2 gas injection.
Fig. 8 depicts (a) the gas injection signal, (b) the
gas pressure in the pipe, (c) the current control signal for inverter power supply, (d) the effective input
Ar after N gas injecpower to the plasma, and (e) T ex
2
tion. The N2 gas flow rate was 1 slpm. From this
figure, the gas pressure increases when the opening
signal turns to 5 V, which means that N2 gas was
indeed injected into the plasma. Without feedback
Ar falls just after N gas injection to
control, the T ex
2
Ar remains around 5000 K dur4000 K. Then, the T ex
ing the N2 gas injection. After the valve closed, the
Ar recovers to 7000 K. On the other hand, with feedT ex
Ar decreases to 6500 K, and then
back control, the T ex
rapidly recovers to 7000 K in 75 ms. This means that
Ar even after
the FBC-MITP can successfully keep T ex
N2 gas injection.
Ar was investigated as a
The recovery time of T ex
function of N2 gas flow rate, as indicated in Fig. 9.
The recovery time was defined as the time from the
valve opening timing until the temperature recovery
to 7000 K. Without feedback control, the recovery
time rises monotonuously with increasing N2 gas injection because more N2 gas injection causes a more
significant temperature drop. With feedback control,
the recovery time retains almost constant independent of N2 gas flow rate. This implies that FBCMITP has a robustness against disturbance due to gas
injection. This feature of the robustness and keeping
temperature may be useful to powder treatment processing or nanoparticle synthesis.
0.6
0.5
0.4
0.3
0.2
0.1
0.0
W/O FBC
W/ FBC
0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
N2 gas flow rate [slpm]
Fig. 9. Temperature recovery time versus nitrogen gas
flow rate injected.
References
[1] Y. Tanaka, et al., J. Phys. D: Appl. Phys., 41 , 185203,
2008.
[2] Y. Tanaka, et al., Appl. Phys. Lett., 90, 071502, 2007.
[3] Y. Tanaka, et al., Asia Pacific Conf. Plasma Sci. Technol.
APCPST-10, OCC-08, 2010-039, 2010.
[4] Y. Tanaka, Int. Conf. Fluid Dynamics ICFD2010,
No.1149, 2010.